I replied for few mails & It was in my outlook. But after some time my Outlook went to hang, and i couldnt see the replied email in Draft/sentitems/outlook. My Exchange admin has confirmed that the email didnt reach the server. But Still there is a Tag in mail saying that "you have replied" on the email.

Can I know is this a bug?

Hi,

Which version of Outlook are you using? Please first make sure you have installed the latest update for your Outlook.

From the description, this issue only happens with few emails, correct? Are these particular emails sent from a special sender? Or can you find any similarities of these emails?

I haven't heard such a bug in Outlook. Have you tried repairing your Outlook data file in case it's due to data file corruption? If not, you can have a try according to the instructions in the following Kb article:

https://support.microsoft.com/en-us/kb/272227

In addition, we can try open Outlook in safe mode to check if we can reproduce it. To do this, press Windows key + R to open the Run command, type outlook /safe and press Enter. If the problem would be gone in safe mode, we can go to File < Options > Add-ins and disable suspicious add-ins to resolve the issue.

Please let me know the result.

Regards,

Steve Fan
TechNet Community Support